The present invention relates to the generation of frequencies of digital type, particulary for the generation of tones, inter-exchange signalling, equipment testing.
Such frequencies are used in particular for the emission of multi-frequency signals, tones, etc.
Tones result from frequencies by timing, whereas multi-frequency signals, for example those in accordance with the "MF SOCOTEL, R.sub.2 or keyboard" codes, result from the combination of various frequencies.
The signal to be generated is a signal of the general form E=sin 2 .pi. ft.
In the PCM (pulse code modulation) systems, any signal characterised by a frequency f is indicated by a sequence of samples x.sub.n. Each sample x.sub.p is written: x.sub.p =sin 2 .pi. f.sub.p T=sin 2 Nt where p is the rank of the sample and T, equal to 1/F, the sampling period. 1/T=8000 Hz is preferably selected in the PCM systems, which limits f to 4000 Hz, according to Shannon's theorem.
A multi-frequency signal generator device has already been described in the Journal Commutation et Electronique No. 59, October 1977, pages 99 to 115. This generator aims at using the periodicity of the sine function on the interval (0,2 .pi.), considering, on this interval, M values sin .alpha., with .alpha.=(2.pi.i)/M. These M values are numbered and inscribed in a table, or read-only memory. This makes it possible to memorize only M'=M/4 values corresponding to the interval (0, .pi./2). If the samples of the memory are read at frequency F, a sequence of samples indicative of a sinusoid of frequency F/M is obtained.
Read-out of the same memory, but retaining only one value every k values, k being a real number between 0 and M/2, furnishes a sequence of samples representative of a sinusoid of frequency f=kf/M.
However, when k is a mixed number, only the whole part of the number is taken into account for readout.
In other words, one does not therefore always read what one was supposed to read, so that an error in read-out, or a distortion, is introduced. Such a process of read-out causes a poor definition in the sequence of samples having to represent the desired frequency.
It is an object of the present invention to overcoe this drawback.